The use of electromagnetic elevator machine brakes is well known in the art. Referring to prior art FIG. 1 such a typical electromagnetic brake is shown generally as 1 having a pair of brake pads 6, 7 attached to levers 2, 3 which are biased into contact with brake drum 8 by a pair of springs 20, 21 to produce sufficient force to hold an elevator car (not shown). In operation, an electromagnetic brake coil 9 lifts brake pads 6, 7 away from drum 8 and thereby releasing the elevator car. A mechanical switch 19 produces a signal corresponding to the position of the brake pads 6, 7, either in the braking position against brake drum 8 or in the lifted position away from brake drum 8. A pair of adjustment screws 22, 23 is further provided within sensing arms 13, 14 to permit adjustment of mechanical switch 19 to allow for wear of the brake pads and other moving and rotating parts.
It is important to monitor the position of the brake pads to ensure that the pads are lifted from the drum prior to the start of motion of the car. If the pads are left in contact with the drum while the elevator car is moving they will wear excessively and require more frequent maintenance. In addition, if the brake drum and pads are heated excessively the electromagnetic brake will lose its effectiveness in braking the elevator car.
The switch system of the prior art as described hereinabove has several drawbacks. The electromechanical switch is actuated every time the elevator car moves subjecting all the moving parts, including the electrical contacts, to deteriorate through wear and tear. Because of the wear and tear the electromechanical switch can go out of adjustment and can cause both false positive signals and false negative signals. If the switch produces a false negative signal, one that indicates that the pads are dragging while the car is moving while they are in fact not contacting the drum, a perfectly normally operating elevator will be shut down for a maintenance action. If the switch produces a false positive, one that indicates that the brake pads have lifted off the drum when in fact they maintain contact with the drum while the car is moving, then the pads will wear and cause excessive heat and reduced braking effectiveness.
Another attempt to monitor elevator brakes is disclosed in U.S. Pat. No. 5,419,415 ('415) wherein a sensor is imbedded within a carrier disposed in the brake pad. The sensor monitors brake pad wear and through a monitoring circuit detects brake lining failures. The sensor in the '415 patent can also be a temperature dependent resistor wherein a temperature within the brake lining can be determined. The system described in '415 has several drawbacks, one being that the integrity of the brake lining is compromised by the placement of the carrier/sensor within the lining itself. The compromise in the structural integrity of the lining may lead to premature failure of the lining and severally degrade brake performance leading to further brake maintenance actions. In addition, the sensor described in the '415 patent requires complicated control circuitry to interpret the change in resistance and correlate it to an increase in temperature. It is also important to note that the circuit disclosed is fairly complex and requires a number of offset voltage sources and reference resistors. Another problem in thermally detecting brake drag in an elevator machine brake is that the ambient temperature within the elevator room may have as high as a 35 degree Celsius deviation within a single day. The '415 patent discloses a temperature sensing circuit which does not monitor ambient conditions and may produce a false signal indicating a brake drag condition simply due to an increase in ambient temperature. In addition the '415 patent teaches the use of a temperature dependant resistant as a sensor which may not provide a rapid enough indication of an actual brake drag condition.
The brake drag monitoring systems of the prior art also have the disadvantage of being incapable of retrofit into existing elevator installations. As such there is a need for an accurate device to monitor and determine brake drag which eliminates or reduces the problems of the prior art.